Unremovable linked nodal structures protected by crystalline symmetries in stacked bilayer graphene with Kekulé texture

Abstract

© 2022 American Physical Society. Linking structure is a new concept characterizing topological semimetals, which indicates the interweaving of gap-closing nodes at the Fermi energy (EF) with other nodes below EF. As the number of linked nodes can be changed only via pair creation or pair annihilation, a linked node is more stable and robust than ordinary nodes without linking. Here we propose a type of linked nodal structure between a nodal line (nodal surface) at EF with another nodal line (nodal surface) below EF in two-dimensional (three-dimensional) spinless fermion systems with IT symmetry where I and T indicate inversion and time-reversal symmetries, respectively. Because of additional chiral and rotational symmetries, in our system, a double band inversion creates a pair of linked nodes carrying the same topological charges, and thus the pair is unremovable via a Lifshiftz transition, which is clearly distinct from the cases of the linked nodes reported previously. A realistic tight-binding model and effective theory are developed for such a linking structure. Also, using density-functional-theory calculations, we propose a class of materials, composed of stacked bilayer graphene with Kekulé texture, as a candidate system hosting the linked nodal structure.